PolymerChemistry

PAPER

Cite this: Polym. Chem., 2019, 10,5080

Received 22nd July 2019,Accepted 12th August 2019

DOI: 10.1039/c9py01086f

rsc.li/polymers

Light, temperature, and pH control ofaqueous azopyridine-terminatedpoly(N-isopropylacrylamide) solutions†

Hao Ren,a Xing-Ping Qiu,b Yan Shi, c Peng Yang a and Françoise M. Winnik *d,e

Azopyridines (AzPy) act as light-sensitive groups that undergo reversible cis–trans isomerization upon UV

irradiation, as hydrogen-bond acceptors, and as ionizable moieties. The kinetics of the thermal cis- to

trans-AzPy deactivation are slow except when the Py nitrogen is H-bonded or cationic. The properties of

AzPy were used here to control the phase transition of aqueous solutions of α-azopyridine-ω-n-dodecyl-poly(N-isopropylacrylamides) (C12-PN-AzPy) with the molar mass (Mn) ranging from 5800 to 19 700

g mol−1. The C12-PN-AzPy polymers form cationic star-micelles in solutions of pH 3 and flower-micelles

in neutral and basic solutions. This diversity of micelle morphology underlies the temperature-, pH- and

UV-irradiation-driven phase transition of aqueous C12-PN-AzPy solutions as demonstrated by turbidime-

try, 1H NMR spectroscopy, and microcalorimetry. Unlike azobenzene, the commonly used photorespon-

sive moiety to actuate amphiphilic polymers, AzPy can affect the thermoresponsive behavior of polymers

in response to three orthogonal triggers: pH, through changes in ionization; light, via trans–cis photo-

isomerization; and time, from hours to a few ms, via the kinetics of the dark cis–trans relaxation. The

study leads the way to responsive sensors or actuators in the form of aqueous fluids, hydrogels, or films

by the application of light and changes of temperature and pH in permutable sequences.

Introduction

Aromatic azo compounds form an important class of dyes andpigments used for centuries as brilliant dyestuffs for textiles,paints, and many other applications.1 In the 1970s, polymerscontaining azobenzenes, commonly referred to as “azo-poly-mers”, caught the attention of photochemists and polymerscientists alike, driven by the pioneering work of Neckersamong others.2 Major advances within the field of azo-poly-mers occurred with the emergence of the concept of azoben-zenes as photosensitive “triggers” of macroscopic changes ofthe properties of materials and solutions, such as phase

changes,3 and photo-induced motion and deformation ofpolymer films.4 In most cases, the photo-response is linked tothe trans-to-cis photoisomerization of azobenzene chromo-phores attached to the polymeric structures. UV-irradiation ofazobenzene causes the conversion of the trans isomer to theless stable cis isomer, which in the dark slowly relaxes to thetrans form. The trans-to-cis photoisomerization of azobenzenechanges the photophysical properties of the chromophore, asdetected by steady-state and transient UV-Vis absorbance spec-troscopy, both of which are well documented.5–7 Other physicalproperties of the azobenzene moiety are affected as well, mostnotably its dipole moment: it increases or decreases uponphotoisomerization, depending on the type and position ofphenyl ring substituents.8,9 Thus, photoisomerization of azo-benzenes linked to a thermosensitive amphiphilic polymeraffects the phase transition temperature of the polymer inwater as a consequence of the different hydrophilicity of the cisand trans isomers.10,11

For practical applications that require fast reversibility, it isadvantageous to use azobenzenes that exhibit an extremelyfast thermal cis to trans relaxation, so that a single UV lightbeam can induce continuous trans ↔ cis isomerizations.12 Therate of the dark cis to trans isomerization of azobenzene can beenhanced significantly through manipulations of the π elec-tron distribution, such as para-substitution of the phenylrings13 or the replacement of one phenyl ring of azobenzene

†Electronic supplementary information (ESI) available: The detail of cloud pointtest influenced by pH values and UV irradiation, as well as the 1H-NMR spec-trum of C12-PN-AzPy 12 K after UV irradiation. See DOI: 10.1039/c9py01086f

aKey Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education,

School of Chemistry and Chemical Engineering, Shaanxi Normal University,

Xi’an 710119, ChinabDepartment of Chemistry, University of Montreal, CP 6128 Succursale Centre Ville,

Montreal, QC, H3C 3J7, CanadacSchool of Materials Science and Engineering, Beijing University of Chemical

Technology, Beijing 100029, P. R. ChinadLaboratory of Polymer Chemistry, Department of Chemistry, PB 55, University of

Helsinki, Helsinki FI00140, Finland. E-mail: francoise.winnik@helsinki.fieInternational Center for Materials Nanoarchitectonics, National Institute for

Material Science, 1-1 Namiki, Tsukuba 305-0044, Japan

5080 | Polym. Chem., 2019, 10, 5080–5086 This journal is © The Royal Society of Chemistry 2019

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by a pyridine ring.14,15 The dark cis–trans isomerization of azo-pyridine (AzPy) in the neutral form is nearly as slow as that ofazobenzene. It is extremely fast (relaxation time ∼1 ms or less)for the quaternized azopyridinium (AzPy+) group. The for-mation of hydrogen bonds between the AzPy nitrogen andH-bond donors also greatly accelerates the thermal cis–transisomerization.

Azopyridine-substituted water-soluble polymers are amphi-philic. They tend to self-assemble in water, adopting variousmorphologies as a function of their chemical composition andtheir environment, in particular the solution pH and the pres-ence of H-bond acceptors.16–18 This phenomenon is exempli-fied in a recent study of aqueous solutions of poly(N-isopropyl-acrylamides) bearing an AzPy on one chain end andan n-dodecyl group on the other end (C12-PN-AzPy)(Scheme 1a).19 In neutral or basic water, they form “flower-like” core–shell micelles with a core of n-dodecyl chains andAzPy groups surrounded by a PNIPAM corona. In solutions ofpH 3, the micelle morphology changes as a result of the proto-nation of the AzPy group. C12-PN-AzPyH+ polymers form starmicelles consisting of a hydrophobic core made solely ofassembled n-dodecyl groups, surrounded by extended PNIPAMchains terminated with the cationic azopyridinium group.

Aqueous solutions of PNIPAM and its derivatives undergo areversible phase transition upon heating past a temperature of∼32 °C that corresponds to the coil-to-globule collapse of thepolymer chains and subsequent aggregation of the dehydratedPNIPAM globules into larger mesoglobules.20–22 This propertyis exploited commonly to actuate temperature sensitivematerials. The phase transition temperature of PNIPAM deriva-tives in water can be adjusted over a broad range of tempera-tures by linking to PNIPAM hydrophilic/hydrophobiccomonomers22–24 or end groups.25,26 The introduction ofstimuli sensitive components responsive to stimuli, such aslight,27 pH,28,29 salinity, redox conditions,30,31 or CO2,

32

expands even wider the range of possible applications ofPNIPAM containing polymers. End-modified PNIPAM deriva-

tives are particularly useful as they can drive large changes ofthe physical or chemical properties of the materials throughminor changes of the polymer chemical composition.33

Generally, non-reactive chain ends do not affect the phasetransition temperature of PNIPAM of Mn ≥ 50 kDa. In the caseof shorter PNIPAM chains, as a rule, hydrophilic end groupsincrease the solution phase transition temperature and hydro-phobic end groups decrease it.

34 These effects result fromchanges in the hydrophilic/lipophilic balance of the modifiedpolymer chains. Charged end groups, such as carboxylates35,36

and quaternary ammonium salts.37,38 tend to prevent theaggregation of the dehydrated PNIPAM globules throughelectrostatic stabilization. If the size of carboxylated PNIPAMglobules is sufficiently small, compared to the wavelength oflight, the solution remains clear at temperatures higher thanthe phase transition temperature.

We report here a study by microcalorimetry, turbidimetry,and NMR spectroscopy of the phase transition of aqueousC12-PN-AzPy solutions as a function of changes in temperatureand solution pH, and upon UV-light irradiation. The studydemonstrates also that through their ionizability andpH-/H-bond-dependent dark relaxation kinetics, azopyridinegroups are simple, versatile, and powerful tools to modulatethe temperature-induced phase transition of amphiphilic poly-mers, such as PNIPAM upon the application of two or threeorthogonal parameters, a key outcome of this investigation.

Experimental sectionMaterials

Water was deionized using a Millipore Milli-Q system. TheC12-PN-AzPy samples were prepared by reversible addition–fragmentation transfer radical polymerization using a chaintransfer agent bearing an n-dodecyl group and an azopyridinemoiety, as described in detail previously.19 Their molar massranges from Mn 5800 g mol−1 to 19 700 g mol−1 (Table 1).Their chemical structure is presented in Scheme 1.

Methods

Turbidity measurements. The cloud point temperatures ofpolymer aqueous solutions (1.0 mg mL−1) were determinedusing a UV-Vis spectrometer (Agilent 8452A) equipped with atemperature controller (Agilent 89090A). The heating rate was0.5 °C min−1. Samples were equilibrated for 2 min at eachtemperature before measurement. The solution transmittanceat 550 nm was measured as a function of the temperaturefrom 15 °C to 60 °C. To determine the cloud point of polymeraqueous solutions subjected to UV light irradiation, the sameprotocol was performed with solutions subjected to constantirradiation at 365 nm with a LED-UV curing lamp (10% inten-sity, 30 mW cm−2). The cloud point (Tcp) was taken as thetemperature of the onset of the decrease of the solutiontransmittance.

Differential Scanning Calorimetry (DSC) measurements wereperformed on a VP-DSC microcalorimeter (MicroCal Inc.) with

Scheme 1 (a) Chemical structure of the polymers investigated in thisstudy; (b) schematic representation of the azopyridine end group ofC12-PN-AzPy in solutions of pH 3, 7 and 10.19

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a cell volume of 0.520 mL. Three consecutive heating andcooling scans from 10–70 °C were performed with a heatingand cooling rate of 1.0 °C min−1 using polymer solutions inwater at a concentration of 1.0 mg mL−1.

1H NMR spectra were recorded on a Bruker AMX-400(400 MHz) spectrometer. Polymer solutions in D2O (2.0 mg mL−1)were brought to pH 10 by the dropwise addition of a 0.1 MNaOD solution. The samples were kept in a refrigeratorfor 24 h prior to measurement. For irradiation-dependent1H NMR analysis, the polymer solutions were irradiated with aLED-UV curing lamp (365 nm, 10% intensity, 30 mW cm−2) for30 s immediately prior to measuring their 1H NMR spectrum.

Evaluation of the photo/pH/temperature responsiveness of thepolymer solutions

The pH- and photo-controlled thermoresponsive properties ofpolymer solutions were evaluated on a Mettler Toledo UV-Visspectrophotometer (UV5 bio) equipped with a CuveT tempera-ture controller. For experiments at constant temperature in theabsence of UV light illumination, the absorbance of a solutionof C12-PN-AzPy (1.0 mg mL−1) at 550 nm was recorded duringa series of consecutives changes of the solution pH from pH 7to pH 3 and from pH 7 to 10 using, respectively, 0.2 mM HClor 0.2 mM NaOH aqueous solutions to adjust the solutionpH. The same measurements were performed also forpolymer solutions under constant UV irradiation (365 nm,30 mW cm−2) at a constant temperature.

Results and discussion3.1 pH Dependence of the phase transition of aqueous trans-C12-PN-AzPy solutions

Table 1 presents key parameters of the heat-induced phasetransition of C12-PN-AzPy aqueous solutions as a function ofpH and the molar mass of the polymers in the absence of UVlight irradiation. The Tcp values of aqueous C12-PN-AzPy solu-tions of pH 7 increase with increasing polymer molar mass(Fig. 1a), as a consequence of the increase of the hydrophilicpolymer chain length while the length of the hydrophobic endgroup remains constant.26,39,40 The Tcp of C12-PN-Azo 12 K, acontrol sample that has an azobenzene end group instead ofthe AzPy group, is nearly identical to that of C12-PN-AzPy 12 K

(22.5 °C vs. 22.7 °C), see Fig. S1.† The cloud points ofC12-PN-AzPy solutions of pH 3, for which the azopyridine endgroup is protonated (pKa of azopyridine ∼4.52),41 are higherthan those of the corresponding neutral solutions, as seen inFig. 1b for solutions of C12-PN-AzPy 7 K. The amplitude of theincrease in Tcp upon acidification, ΔTcp = Tcp (pH 3) − Tcp (pH 7)

decreases with increasing polymer molar mass (see Fig. 1c, redbars). The Tcp of C12-PN-AzPyC2H5

+ 12 K, which bears the cat-ionic N-ethyl azopyridinium end group, is (28.4 °C, Fig. S1†) avalue similar to that of C12-PN-AzPy 12 K in a pH 3 solution(28.2 °C). As a comparison, the Tcp values of aqueous solutionsof unmodified PNIPAM of pH 3, 7, and 11 were determined aswell (Fig. S2†). The Tcp values of the pH 3 and 11 solutionswere ∼0.5 °C lower than those of the pH 7 solution. It is worthmentioning that the amide group of the NIPAM repeat unitsand the ester group that links the AzPy end group to the main

Table 1 Molar mass data of the polymers examined here and properties of their aqueous solutions (pH 7 and pH 3, polymer concentration:1.0 mg mL−1)

Sample name Mn (g mol−1) a Mw/Mna

Tcp/°C Tm/°C ΔH/kJ

pH = 7 pH = 3 pH = 7 pH = 3 pH = 7 pH = 3

C12-PN-AzPy 5 K 5800 1.25 14.0 23.9 18.9 —b 2.7 —b

C12-PN-AzPy 7 K 7800 1.03 20.0 27.5 24.2 26.2 3.2 3.9C12-PN-AzPy 12 K 12 900 1.09 22.7 28.2 26.8 28.1 4.3 4.9C12-PN-AzPy 20 K 19 700 1.03 27.0 32.4 29.2 30.1 6.0 7.3C12-PN-AzPyC2H5

+ 12 K 12 900 1.09 30.0 28.4 31.1 31.0 4.7 4.2C12-PN-Azo 12 K 13 600 1.23 22.5 19.5 26.6 —b 4.2 —b

a Mn and Mw/Mn were obtained by GPC with MALS detection (see ref. 14). bData not available.

Fig. 1 (a). Plots of the % transmittance at 550 nm as a function oftemperature for solutions of C12-PN-AzPy of various molar mass inwater (pH 7); (b) plots of the % transmittance as a function of tempera-ture for solutions of C12-PN-AzPy 7 K of pH 7 (black squares) and pH 3(red circles). ΔTcp is defined as Tcp (pH 3) − Tcp (pH 7); (c) ΔTcp (red bar)and ΔTm (blue bar) values for C12-PN-AzPy of various molar mass;(d) thermograms of solutions of C12-PN-AzPy 7 K of pH 7 and pH 3,ΔTm is defined as Tm (pH 3) − Tm (pH 7).

Paper Polymer Chemistry

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chain withhold the experimental conditions, as reported pre-viously according to previous reports.19,42

High sensitivity microcalorimetry (HS-DSC) thermogramsrecorded for aqueous solutions of C12-PN-AzPy 7 K of pH 7.0(black squares) and pH 3.0 (red circles) are presented in Fig. 1d,where Tm denotes the temperature corresponding to themaximum of the endotherm. The shift to a higher temperatureof the transition maximum upon acidification (0.9 °C < ΔTm <2.0 °C, with ΔTm = Tm (pH3) − Tm (p H7)) (Fig. 1c, blue bars) issmaller than ΔTcp. The enthalpy of the phase transition (ΔH,expressed in kJ mol−1 of NIPAM units) decreases with decreas-ing C12-PN-AzPy molar mass, especially in the case of neutralsolutions (Table 1). This trend was observed also in earlierstudies of aqueous solutions of neutral α, ω di-n-octadecyl-PNIPAM that forms core–shell flower micelles.43,44 Since thetransition enthalpy corresponds to the release of polymer-boundwater molecules into bulk water,45 a decrease in enthalpy indi-cates a lower degree of hydration of the PNIPAM corona,especially in the corona section close to the alkyl core as a resultof the chain confinement. The ΔH values recorded for aqueoussolutions of the longest polymer, C12-PN-AzPy 20 K in neutraland acidic solutions, 6.0 kJ mol−1 and 7.3 kJ mol−1, respectively,are similar to those of unmodified linear PNIPAM,44,46 Hence,C12-PN-AzPy 20 K exists in water in the form of free chains orloose aggregates, rather than core–shell micelles.

Comparing the turbidity curve recorded for the aqueousacidic C12-PN-AzPy 7 K (Fig. 1b, red curve) to the thermogramof this solution (Fig. 1d, red curve), we observe that the Tcpvalue of this solution, 27.5 °C, is higher than Tm (26.2 °C). Theazopyridinium ions stabilize the dehydrated/collapsed chainsagainst macroscopic aggregation,36 but their stabilizing effectis confined within a narrow temperature window (∼1–2 °Cabove T ). Upon further heating, solutions become turbid indi-cating the formation of large mesoglobules. We surmise, thatas the temperature increases, the hydrophobic AzPy+ groupsrelocate within the collapsed chains decreasing the electro-static repulsion between collapsed globules. In the case of theneutral C12-PN-AzPy (pH 7) solutions, Tcp nearly coincideswith the endotherm onset temperature, which confirms thatunder neutral conditions the clouding of the solution and thedehydration/collapse of the PNIPAM chains occur at about thesame temperature (Fig. 1d). The Tcp values of polymer solu-tions of pH 10 are slightly lower than the corresponding valuesfor solutions of pH 7, which can be attributed to the presenceof OH− ions.47,48 (see turbidity plots in Fig. S3†).

3.2. Combined effects of pH and UV light irradiation on thephase transition of aqueous C12-PN-AzPy solutions

UV light photoirradiation of trans AzPy yields the less stable cisAzPy that thermally relaxes back to the trans form. Asdescribed above, the cis–trans thermal relaxation of AzPyH+ inC12-PN-AzPyH+ micelles (pH 3) is on the order of a few ms asdetermined by transient absorption spectroscopy (Scheme 1b,box pH = 3 inset). In neutral solutions, the cis–trans dark relax-ation is on the order of a few seconds, since the flower-likemorphology of C12-PN-AzPy enables the formation of H-bonds

between the AzPy nitrogen and the amide hydrogen of the sur-rounding PNIPAM repeat units. In a basic solution, the darkcis to trans relaxation of AzPy takes several hours since thehydrogen bonds between the AzPy nitrogen and the PNIPAMamide groups are broken by the excess hydroxyl ions compet-ing with the AzPy nitrogen as hydrogen bond acceptors.19

Cis-C12-PN-AzPy is sufficiently long-lived in alkaline aqueoussolutions for the characterization by NMR spectroscopy, turbi-dimetry, and steady state absorption spectroscopy (Fig. S4†).

The 1H NMR spectra of C12-PN-AzPy in D2O (pH 10) beforeirradiation and after 30 s irradiation at 365 nm are presentedin Fig. 2a and b, respectively. Then trans-to-cis isomerizationinduces significant changes of the aromatic proton reso-nances. The protons meta to the pyridine nitrogen and theprotons on the phenyl ring experience large upfield shiftsfrom 7.79, 7.54, and 6.95 ppm (labeled b, c, and d) to∼6.86 ppm. The protons ortho to the pyridine nitrogen(labeled a in Fig. 2) undergo a small upfield shift from 8.63 to8.45 ppm. In addition, the total area of the signals due to thearomatic protons, normalized to the area of the signal at3.85 ppm due to the NIPAM methine protons increases by48.3% (from 0.060 for trans-C12-PN-AzPy to 0.089 for the cisisomer). The upfield shift of the aromatic proton resonancesreflects the decrease of the π-electron delocalization in the nonplanar cis AzPy, compared to trans AzPy.37 The apparentincrease of the aromatic resonance intensity is an indicationof the enhanced mobility of the aromatic rings of cis-AzPycompared to trans-AzPy, possibly as a result of a higher level ofhydration of the cis AzPy vs. the trans AzPy. Similar 1H NMRresults were observed in aqueous solutions of C12-PN-AzPy12 K (Fig. S5†).

The cloud points of pH 10 aqueous solutions of cis-C12-PN-AzPy were obtained by measuring the changes in the solu-tion transmittance at 550 nm as a function of temperatureunder constant irradiation at 365 nm. The resulting turbidityplots of solutions of cis-C12-PN-AzPy 7 K (full symbols) andtrans-C12-PN-AzPy 7 K (open symbols) are presented in Fig. 3b.The cloud point of the cis form is significantly higher than

Fig. 2 1H-NMR spectrum of C12-PN-AzPy 7 K (a) before and (b) afterUV irradiation at pH = 10 (1.0 mg mL−1 in D2O, 18.5 °C).

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that of the trans form, with ΔThν = Tcp (cis) − Tcp (trans) ∼5.8 °C. The ΔThν value decreases with increasing PNIPAMmolar mass, due to the increase of the NIPAM units/AzPy ratio

(see Table 2) presented in Fig. S6.† The increase in thehydration, observed by 1H NMR spectroscopy, and largerdipole moment of cis-AzPy, compared to trans-AzPy, are contri-buting factors causing the increase of Tcp (cis).

49

We measured also turbidity plots of C12-PN-AzPy aqueoussolutions of pH 3 and 7 before irradiation (trans form) andunder continuous UV irradiation. For solutions of pH 3, theTcp values recorded during irradiation were identical to thecorresponding Tcp (trans), as anticipated since the short-livedcis isomer cannot be detected by turbidity measurements. TheTcp of C12-PN-AzPyC2H5

+ during irradiation, also, is identicalto that recorded for trans-C12-PN-AzPyC2H5

+ (see Table 2). Asmall increase of Tcp upon irradiation was observed for C12-PN-AzPy in neutral aqueous solutions, which indicates that theirradiated solutions contain a small, but detectable, fraction ofcis-C12-PN-AzPy throughout the measurement.

Scheme 2 presents an overview of the properties andappearance of aqueous solutions (pH 3, pH 7, and pH 10) of

Fig. 3 (a) ΔThv values for C12-PN-AzPy of various molar mass at pH 3 (red bar), pH 7 (black bar) and pH 10 (blue bar), where ΔThν = Tcp (underirradiation) − Tcp (before irradiation). (b) Plots of the % transmittance as a function of temperature for solutions of C12-PN-AzPy 7 K of pH 10 before(open squares) and under continuous UV irradiation (full squares) at 365 nm.

Table 2 UV induced cloud point shift for the polymers under differentpH values, where ΔThν = Tcp (under irradiation) − Tcp (before irradiation)(polymer concentration: 1.0 mg mL−1)

Sample name pHTcp beforeUV/°C

Tcp afterUV/°C ΔThv/°C

C12-PN-AzPy 7 K 3 27.5 27.1 −0.47 20.0 22.6 2.6

10 16.0 21.8 5.8C12-PN-AzPy 12 K 3 28.2 28.4 0.2

7 22.7 24.1 1.410 22.0 24.3 2.3

C12-PN-AzPy 20 K 3 32.4 32.0 −0.47 27.0 28.5 1.5

10 26.8 28.2 1.4C12-PN-AzPyC2H5

+ 12 K 7 30.0 30.4 0.4C12-PN-Azo 12 K 7 22.5 24.5 2.0

Scheme 2 pH- and UV-induced thermoresponsive properties of C12-PN-AzPy (the temperatures correspond to a polymer of Mn ∼ 7000 g mol−1,ΔOD: difference in the optical density before and after irradiation).

Paper Polymer Chemistry

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AzPy capped PNIPAMs upon UV irradiation and/or uponheating and the different time scales of the phenomenainvolved.

In neutral or basic aqueous solutions of C12-PN-AzPy (blueand green frames) the polymers adopt a core–shell micellemorphology. Upon heating in the dark, the PNIPAM chains ofthe shell dehydrate and collapse resulting in the formation ofsmaller hydrophobic micelles that aggregate into larger par-ticles, observed visually by the appearance of turbidity (top rowvials in each frame). Upon short UV irradiation, the turbidsolutions become transparent, since light converts trans-C12-PN-AzPy into cis-C12-PN-AzPy, which has a higher Tcp. In thedark, solutions of pH 7 become turbid again within a fewseconds. The recovery is very slow in the case of solutions ofpH 10.

In solutions of pH 3 (yellow frame), C12-PN-AzPyH+ chainsassemble in star cationic micelles. The heat induced dehydra-tion of PNIPAM results in the formation of small aggregates ofcollapsed micelles stabilized electrostatically, barely detectablevisually, that increase in size upon further heating.36 ConstantUV irradiation has no effect on the visual appearance of pH 3C12-PN-AzPyH+ solutions at a given temperature, since thetrans and cis isomers undergo fast exchange.

Based on the above, we can design complex “isothermal”phase transition systems by simply changing the solution pHor applying UV irradiation. For example, we can control thesolution turbidity at a constant temperature by consecutiveschanges of the solution pH from pH 7 to pH 3 or to pH 10using 0.2 mM HCl or 0.2 mM NaOH aqueous solutions, seeFig. S7.† An increase of the pH from 7 to 10 will instantly stopthe UV on/off reversible isothermal transition, and the reversi-bility is recovered upon decreasing the solution pH from 10 to7. Various permutations of the trigger sequences can be envi-saged in specific situations.

Conclusion

The ionization of AzPy end groups, the trans–cis photoisomeri-zation of AzPy, and the kinetics of the cis–trans relaxation overtime ranging from hours to a few ms influence the thermo-responsive behavior of the polymers, each in a specific way.Ionization of the azopyridine end group stabilizes the de-hydrated chains against macroscopic aggregation within aspecific temperature range (∼1–2 °C above Tm). Of particularimportance from the practical view point, the UV induced tur-bidity of the polymeric solutions changes as a function of pH,as a consequence of the pH-dependent kinetics of the cis totrans dark relaxation of AzPy. This study contributes to the fun-damental understanding of the effects of end group ionizationand photosensitivity on the macroscopic phase transition ofmodified PNIPAMs. It leads the way to simple strategies for theuse of AzPy functional amphiphilic polymers in aqueousfluids, hydrogels, or films as sensors or actuators controllableby the application of light and changes of temperature and pHin permutable sequences.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

HR. thanks the China Postdoctoral Science Foundation Grant(No. 2019M653859XB). FMW acknowledges the NaturalSciences and Engineering Research Council of Canada forpartial support of this work.

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